1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to feedback controlled elevator systems which compare a signal responsive to the actual speed of an elevator car with a speed pattern signal.
2. Description of the Prior Art
U.S. Pat. No. 4,161,235, which is assigned to the same assignee as the present application, discloses an elevator system in which predetermined parameters of the speed pattern signal, such as its rate of change, and its peak or maximum value, are monitored and compared with predetermined reference values. The exceeding of a reference value by a monitored parameter results in the clamping of the speed pattern such that it cannot exceed about 1.1 times the normal acceleration rate, and about 1.01 times the normal full speed. Slope limiting of the speed pattern, and smoothly blending the acceleration and full speed portions of the pattern, accomplished while limiting the maximum value of the speed pattern, are also disclosed.
In order for the floor selector of an elevator car to make proper, timely decisions, such as when to initiate the slowdown portion of the speed pattern signal, the position of the elevator car must be known to the floor selector. An arrangement suitable for solid state controls is disclosed in U.S. Pat. No. 3,750,850, which is assigned to the assignee of the present application. This solid state selector arrangement determines car position by incremental counting. Pulses are generated by a pulse wheel as the elevator car moves in the hatch, and these pulses increment a car position counter up or down, depending upon the travel direction assigned to the car by the floor selector. This arrangement works very well, but is subject to certain errors, such as when the elevator car "rolls back", i.e. moves in a direction opposite to the assigned travel direction. Pulses generated during this slight movement will cause the counter to be incremented in the wrong direction. Noise pulses also cause errors, as does wear of the apparatus driving the pulse wheel, such as cable wear and sheave wear. Errors may also be due to governor cable creepage on the sheave which drives the pulse wheel. Loss of power also causes the position counter to lose its count.
Finally, an elevator system, such as the one disclosed in U.S. Pat. No. 4,161,235, requires a completely independent, redundant terminal slowdown and stopping arrangement for each elevator car. The function of this arragnement is to slow down and stop the car automatically at the top and bottom terminal landings, should the normal slowdown and stopping arrangement fail to function properly. U.S. Pat. No. 3,779,346, which is assigned to the same assignee as the present application, sets forth an example of an independent terminal slowdown control which may be used. Regardless of the type of independent terminal slowdown arrangement, it usually involves a certain amount of hatch equipment mounted near each terminal floor, and additional processing circuitry. In these arrangements, the need for independent terminal slowdown operation must first be detected, and then the speed pattern is modified to cause the car to make a reasonable approach to the terminal floor.